Antennas are normally connected to a radio by a direct galvanic connection. However, it has been shown that feeding an antenna through a capacitive gap (e.g. between a conductive strip and a feeding structure) can provide several advantages for certain types of antenna. The advantages are particularly useful for larger impedance matching bandwidth. See, for example, U.S. 2003/0189625 or Rowell & Murch, “Compact PIFA Suitable for Dual-Frequency 900/1800-MHz Operation”, IEEE Transactions on Antennas and Propagation, Vol. 46, No. 4, April 1998, pp. 596-598.
The single band antenna shown in FIG. 1 of Rowell & Murch has a wide feeding plate that extends across a slot formed in the main antenna element. The dual band antenna shown in FIG. 2 has a separate antenna element and a separate capacitive feed for the upper frequency band of the antenna. It is clear that the authors of this paper have not considered the possibility of creating multiple resonance behaviour with a single antenna element and a single capacitive feed.
EP1345282 discloses a multiband radio antenna device (1) for a radio communication terminal, comprising a flat ground substrate (20), a flat main radiating element (2,9) having a radio signal feeding point (3), and a flat parasitic element (5,6). The main radiating element is located adjacent to and in the same plane as the ground substrate, and preferably dielectrically separated therefrom. The antenna device is suitable for being used as a built-in antenna in portable radio terminals, such as a mobile phone (30). However, it is to be noted that this antenna is not a capacitively fed antenna. In EP1345282, the feeding element is also the longest element and the one that gives the lowest resonant frequency as well as the multiband behaviour; the antenna would still work at the same lowest resonance if the capacitively coupled element were removed.
EP2405533 discloses a capacitively fed antenna including an inductive element (181) that is required to create the multiband resonance behaviour of the antenna. Moreover, the feeding element shown in EP2405533 is configured so as to start at a point remote from the grounding point of the antenna and to run towards the grounding point in the opposite direction to that of the radiating arms of the antenna.
US2012/0154222 shows an antenna structure comprising a long, U-shaped element and a shorter, inverted L-shaped element. Here, the U-shaped element is driven and the L-shaped element is shorted to ground.
FIG. 1 of the present application illustrates a known capacitively fed antenna. The antenna 102 is connected to the ground plane 106 and folded at point A so that at least part of the antenna is in a plane substantially parallel to the ground plane 106. Folding the antenna in this manner reduces the overall height of the antenna device. The antenna 102 is connected to the ground plane 106 at the grounding point 108. The radio transmitter/receiver 110 is connected to the feeding structure 104, and a small capacitive gap 112 is formed between the feeding structure 104 and the antenna 102. The capacitance of the capacitive gap 112 is a design parameter and depends on the frequency of operation. For example, the capacitance of the gap 112 could be approximately 2 pF for a frequency of operation of around 1 GHz.
Typically the capacitive gap 112 is positioned close to the grounding point 106 of the antenna 102. In this configuration, the impedance of the antenna at the capacitive gap 112 is close to the characteristic impedance of the radio system, for example, 50.
The antenna illustrated in FIG. 1 is typical of capacitively fed antenna devices, however there are various ways in which the overall size of the antenna may be reduced by folding the antenna. Furthermore, it is possible to create multiple resonances by the addition of branches on the antenna 102. It should be noted that the antenna device illustrated in FIG. 1 is an unbalanced structure and the ground plane 106 of the antenna device is an integral part of the radiating structure and plays a major role in the overall performance of the antenna device.
The type of structure illustrated in FIG. 1 is widely used in many devices (e.g. cellular antenna for mobile phones, laptops, etc.) and many variations are disclosed in the prior art.